1º- Delete your save;


2º- Start a new game;


3º- Get to the arena of Great Grey Wolf Sif and DON'T KILL HIM;


4º- He is alive again :), is like you never killed him.


And after doing all this steps your sadness/guilt is way gone.

If you still feel guilty and sad for killing him follow this steps:

1º- Go to the DLC;


2º- Go to the Chasm of the Abyss;


3º- Follow the Spirit of Alvina (the cat);


4º- Attack the wall where Alvina diseapears;


5º- KILL ALL THE GIANT HUMANITIES TO SAVE SIF!!!!


And that's it.
You saved Sif.
You should feel less guilty and sad.